Sweetness, a taste typical of sugar, is one of the five primary tastes, and saccharides such as sugar and glucose are primarily used as natural sweeteners. Also in the case of amino acids, proline, glycine and alanine are known to be sweet and may also be used for enhancement of sweetness, as circumstances demand, because the sweetness quality of these amino acids is different from that of saccharides.
Likewise, hydrolyzed animal proteins used frequently as seasonings in processed foods may also be used for enhancement of sweetness, because such hydrolysates mainly originate from collagen or gelatin, and hence have a characteristic sweetness rich in proline, glycine and alanine which are constituent amino acids of collagen or gelatin.
Yeast extracts are also used as seasonings in various foods (Patent Documents 1 to 3). For the application of yeast mutant strains in the field of food industry, some reports have been issued. For example, there is a report showing that mutant strains tolerant to AZC (a toxic analog of proline) have been screened to obtain a mutant strain which allows intracellular proline accumulation and is more tolerant to freezing stress than its parent strain (Non-patent Document 1). According to the same procedures or the like, a freeze-tolerant yeast strain which allows intracellular accumulation of one or more amino acids selected from proline, arginine, lysine and glutamic acid has been used in frozen bread dough (Patent Document 4). Likewise, a proline-accumulating yeast transformant has been used for brewing of Japanese rice wine (Patent Document 5). However, the former report focuses on the survival rate of yeast in frozen bread dough, while the latter report is directed to the ethanol tolerance of yeast and/or modification of Japanese rice wine to have a milder taste. Neither of them provides any information about the use of yeast as a seasoning. Moreover, neither of them aims to enhance a hydrolyzed animal protein-like sweetness or provides any information about the overall sweetness of proline, alanine and glycine.    Patent Document 1: JP 2005-102549 A    Patent Document 2: JP 10-327802 A    Patent Document 3: WO99/16860    Patent Document 4: JP 9-234058 A    Patent Document 5: JP 2006-67806 A    Non-patent Document 1: H. Takagi, F. Iwamoto, and S. Nakamori, Appln. Microbiol. Biotechnol., 47, 405-411, (1997).